| The north of margin in Qaidam Basin has a complex tectonic evolution history,many eclogite have been found in this area since the late 1990s,as the research going,multi-type high pressure and ultrahigh pressure rocks come to light,furthermore,based on the detailed chronological research,this area has been confirmed that it is a ultrahigh pressure metamorphic belt experienced continental crust subduction and collision during the early Paleozoic.In this ultrahigh pressure metamorphic belt developed in a mass of the Paleozoic to the Mesozoic granite.The Saishiteng Mountain is a typically rock mass which located in the north margin of Qaidam Basin.XPF and ICP-MS were used to study geochemistry of the geology,petrotectonics and former studies to discuss the rock type,formation ages,material source and tectonic background,the main results are as follows:?1?Classsification by geochemistry the granites which outcropped on the Saishiteng mountain belongs to acidic rocks,show signs of granite and granodiorite in nomenclature diagram of granitoids,metaluminous-weak peraluminous in A/NK-A/CNK diagrams for granite;has high content of alkali,the sample setting points dropped in the area of calc-alkaline,belongs to calc-alkaline ones;great variational amplitude of MgO,CaO,FeO;show I type granite in CIPW.As a whole,It suggests that the rock of Saishiteng mountain belongs to metaluminous-weak peraluminous and calc-alkaline I type ones.?2?Trace elements shows that the sample of Saishiteng granites has genetic relationship;rich in large ion lithophile element,poor in high field-strength element;relative enrichment Sr,Ba,K in lithophile.?REE has large amplitude(74.29×10-6169.11×10-6,average value is 124.84×10-6),the fractionation degree of LREE and HREE is low,rich in LREE,differentiation is not obvious in the interior of LREE,curve of HREE is flat,dephlegmation is inconspicuous and less obvious negative Eu.?3?The research of O3T would make good restriction to the arrival age of Saishiteng granites due to the granites intrude into it,based on previous data published,we can assume the rock formed in Ordovician.The time order may be:Kongquegou granites-Hongliugou granites-Baishishan granites,due to field geological survey and character of ore minerals.?4?The specific value of La/Ta is 27.67106.24,much higher than 25;ratio of La/Sm is 6.5217.36,much higher than 5,strong depleted in P.Comprehensive analysis reflect that the granites of Saishiteng mountain were derived from depleted type mantle and that hybridism of continental crust occurs in the source area.?5?The granites enrichment in LREE,average ratio of K2O/Na2O is 0.48,rock type is calc-alkaline,rock association characteristic is gabbro+granite+granodiorite,combine with tectonic discrimination diagram of major and trace elements,result shows that the granite formed in island arc environment,it means that the South Qilian ocean has not closed when the granites emplacement. |
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